As is well known, great effort has been expended in recent years to improve the safety performance of vehicles such as automobiles; as a result, a variety of technical measures have been devised, developed and put into practical use. Among them, gas bag inflation systems have been developed as a very important means for protecting the occupants of motor vehicles from violent shocks in case of collision with other vehicles or stationary objects such as walls. Now, a variety of improved gas bags and their peripheral arts have been devised.
However, due to the requirements of manufacture, materials and manufacturing costs, the gas bags heretofore used have usually been produced by cutting into prescribed sizes a plain-woven fabric that serves as a substrate fabric, superposing the cut fabrics at their margins, and stitching the superposed portions in a plurality of rows along the margins.
That is, according to the conventional art, the cut base fabrics 1, 1 were superposed at preselected margins 2, 2 as shown in FIG. 1 and FIG. 2, and superposed portions 3 were stitched into a plurality of rows of seams 4a, 4b, 4c, 4d,--using stitching yarns 5, 5,--by means of a sewing machine or any other suitable means. Stitched portions of the fabrics usually exhibit a tensile strength which is relatively smaller than that of the base fabric; therefore, the seams are provided in a plurality of rows to cope with the decrease in strength.
According to the aforementioned conventional gas bags, the seams 4a, 4b, 4c, 4d, were stitched maintaining pitches Pa, Pb, Pc, Pd in a relation Pa=Pb=Pc=Pd, i.e. the lengths of the stitches in each row 4a, 4b, 4c, 4d were equal; the seams were simply arrayed spaced laterally and repetitively in order to avoid the decrease in tensile strength.
The mode of breakage at the stitching portions can generally be classified into three types depending upon the density of yarns of the base fabric at the stitching portions, stitch, strength of stitching yarns, and the strength of the yarns forming the base fabric:
(i) The stitching yarn breaks if its strength is less than that of the yarns forming the base fabric between neighboring stitches;
(ii) The yarns forming the base fabric break if the strength is in an opposite relation to that of (i) above, i.e., the base fabric breaks if its yarns have less strength than the stitching yarn; and
(iii) The yarns of the fabric at the stitching portion are loosened and removed if the above said strengths are balanced and a great tensile load is applied to the base fabric.
With gas bags, since the base fabric was cut into a prescribed size as mentioned above, and the cut pieces were stitched together, it was accepted practice to rely on the aforementioned situation (i). In other words, the theoretical background for avoiding tearing of the base fabric of gas bag when the gas was injected to inflate the bag, was based on the aforementioned plurality rows of seams 4a, 4b, 4c and 4d, using stitching yarn weaker than the fabric yarn.
When the seams were stitched maintaining an equal pitch as represented by Pa=Pb=Pc=Pd, however, the first situation (i) developed due to complicated competitive conditions that will be mentioned later. In this case, a maximum of tensile load was first exerted on the outermost seams 4a and 4d, causing the seams 4a and 4d to be broken. Accordingly, the tensile load was not significantly attenuated and instantaneously thereafter an almost equivalent load was exerted on the inner seams 4b and 4c, causing the seams 4b and 4c also to be broken. Consequently, the gas bags often failed to exhibit their functions just like when the bag was stitched using a single seam.
In order to function properly to cushion the passengers from the collision, the bag system must have a rate of gas inflation of about 1/1.000 to 1/10.000 seconds depending upon the conditions, and at this rate the tensile load is very great.
When the event of situation (ii) mentioned above is developed depending upon the conditions, the base fabrics 1, 1 are torn apart at the stitching portion 3, permitting the inflated gas to be injected into the room from the torn-apart portions and spoiling the function of the bag for absorbing shocks.